From what I understand our friends at Gatwick will issue you an FAA Med at the same time as a JAA one. However if you already have your JAA medical I think you will have to sit the FAA exam.
You can get a list of all the FAA AME from the FAA website
http://www.faa.gov/pilots/amelocator/
Remember that you will need form 8500-8, again you can do this online at
https://medxpress.faa.gov/
For FAA medicals there is no initial like ours so all the examinations are to the same standard. You only need a class 1 for ATP, class 2 for CPL and class 3 for PPL. In fact there is little difference between the examinations, but class 1 does have higher eyesight requirements and ECG, expect a cost of about £150.
Hope this helps
Couple of things.
I don't know if the CAA at Gatwick can issue FAA medicals.
You don't have to bring FAA form FAA 8500-8 to your FAA AME; he or she will give you one.
You only need an FAA Class 1 or 2 if you want to execute the privileges of an ATP or CPL; you do not need more than a class 3 for doing any flight test; even an ATP or CPL checkride can be done on a class 3.
Having said that there is little difference between the classes other than, as has been said correctly, some additional vision tests and an ECG for a class 1.
Costs vary between AMEs.
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